There is already known, for instance, from the U.S. Pat. No. 4,341,725, issued on July 27, 1982, a process of molding refractory and metal shapes by slip-casting, wherein a nucleating agent is added to the slip prior to the casting and to the subsequent freezing of the cast preform in order to keep the sizes of ice crystals forming in the cast preform during the freezing of the latter at a level sufficiently low to avoid structural damage to the preform that would result if the sizes of the ice crystals were excessive. While this patent mentions that the shape of the article produced by using this method may be quite intricate, it specifically describes the use of its method only in the context of the making of a simple tube of constant inner and outer diameters. Consequently, the disclosure of this patent is insufficient to indicate how its teachings could be employed in the making of more intricate articles, especially of complicated hollow lightweight articles.
Yet, it is highly desirable to use high stiffness, lightweight articles in many applications, especially but not exclusively in mobile, launched or dynamic applications where specific section stiffness (EI/A.rho.) is a key performance parameter. Additionally, a high degree of thermally induced distortion control is required in some of such applications (e.g. in optical systems) in order to minimize distortion or optical misfigure. These applications typically depend on the selection of materials for the articles so as to have high thermal stability (k/.alpha.). Hence, materials with high elastic moduli, high thermal conductivities, low densities, and low coefficients of thermal expansion are desirable for use at least in such applications.
Currently, the material of choice for such applications has been beryllium (Be), which is typically machined or formed by hot isostatic pressing into open webbed structures. Other materials which, in the same geometries, approach the specific section stiffness (EI/A.rho.) of beryllium, include refractories such as silicon carbides, silicon nitride and similar materials which will be collectively referred to as ceramics. These materials, which are amenable to net shape forming, would surpass beryllium in stiffness and thermal performance if formed into closed-web geometries. Yet, forming closed-web structures in either metals or refractories so far required the use of complex tooling or extensive machining followed by bonding to produce the desired shapes. Such processing resulted in a cost-prohibitive product.
Accordingly, it is the general object of the invention to avoid the disadvantages of the prior art.
More particularly, it is an object of the present invention to provide a method of making lightweight yet sturdy ceramic articles, which method does not possess the disadvantages of the prior art methods of this type.
Still another object of the present invention is so to develop the method of the above type as to be able to utilize it in the manufacture of closed-web ceramic articles.
It is yet another object of the present invention to devise a method of above type which minimizes if not eliminates the development of the stresses in a frozen ceramic preform due to differential thermal expansion.
In keeping with these objects and others which will become apparent hereafter, one feature of the present invention resides in a method of manufacturing a light-weight yet sturdy ceramic article. According to the invention, this method includes first forming at least one fugitive core including a main portion and at least one holding portion projecting from the main portion and then positioning the fugitive core in a mold cavity bounded by a multitude of bounding surfaces so that the holding portion holds the main portion at predetermined spacings from all of the bounding surfaces. Thereafter, the mold cavity around the fugitive core is filled with a dense slip constituted by ceramic particles in mixture with a liquid medium to form a ceramic preform embedding the fugitive core in its interior, the ceramic preform is caused to freeze, and the fugitive core is caused to dissolve and escape from the interior of the frozen ceramic preform. Finally, the ceramic preform is converted into the ceramic article.
Thus, the present invention presents a process capable of forming closed-web structures in refractory or ceramic materials at costs significantly lower than either the previous by proposed closed-web manufacturing processes or the current technique for generating open-web beryllium structures.